Allow to drop frames to keep A/V sync

Description

A user uploaded a HD recording of a Lumia cellphone which shows heavy A/v desync issues with QT because audio has "holes" - especially noticeable after 77 seconds. FFplay, MPlayer and WMP speed up video to keep A/V sync.
FFmpeg by default looses A/V sync, encoding with -async 1 works fine but produces silent moments.
There should be an option to drop video frames to keep A/V sync if another stream is missing content, just like the media players skip these frames.

Change History (5)

Confirmed!
-async 1 gives me the same results as "playing with player and capture it", aka keep the sync at the cost of audio gaps (which is fine for now, aka best result up to now). Ffmpeg method is much better than play-record (especially since I can batch it).
A "drop or insert frames without mercy" variant which keeps the audio gapless would be indeed very nice and preferred by me.
If the "source2.mp4" is needed again for testing just say, it is currently removed from the webspace.